| In order to meet the demands of small shape,high density,and light weight,electronic packaging technology and printed circuit board(PCB)technology was developping at a high speed.Therefore,density of solder joints which provide mechanical support and path of electricity and heat also was increasing.Due to the macro position and internal structure of solder,the failure of electronic products is closely linked to the quality of solder.As a result of failure of one solder joint,the product may fail functionally.It means that the reliability and quality of solder joint have closely connected with the quality and reliability of product which contains high-density solder joints.And the study of quality and reliability of solder joints is becoming more and more important.In this paper,three finite element model was constructed to simulate the reliability of solder during soldering,isothermal aging and working.Then study the solder failure mode by designing some experiments under different thermodynamic conditions to verify the simulation result of solder joints.Solder joint interconnection is typically consist of solder,pad and intermetallic compound(IMC)layer after soldering.Thin,continuous and uniform IMC is a good foundation for metallurgical interconnection.However,if the IMC layer is too thick,it will become a hidden danger of solder joint failure.As the prerequisite of metallurgical interconnect between solder and substrate,interfacial reactions in solder-substrate interface system with different material and structure will lead to generation of IMC with different element and microstructure.The introduction and interaction of these factors will cause uncertain tend of reliability issue of solder joints.In this paper,the finite element model was build to simulate and visualize the solid-liquid diffusion of atom during soldring.Then,the interfacial reaction of SAC305/Cu(or Ni)as well as the formation and growth mechanism of IMC were investigated and evaluated after experiment based on finite element analysis.The result proves that Ni layer can act as the diffusion barrier layer to avoid generation of thick IMC.And possible position of failure can be predicted.Due to the result of residual stress simulation,cracks may appear on the thick IMC layer where the stress was concentrate on the edge of the solder joint.The evolution of size,morphology and structure of IMC is controlled by migration behavior of atom and mass from the layer toward the solder.This evolution may result in new generation of IMC with poor mechanical properties and appearance of KirKendall void or cracks which will provide cracking sources.This evolution can be studied during isothermal aging at 150°C.And a finite element model was constructed to simulate and visualize the diffusion of atom between solid and solid during isothermal aging.According to the result of simulation,it was found that morphology of the IMC after soldering determines the direction of its evolution;Changes in the content of IMC's constituent elements are the main factors that cause evolution of IMC;Metal elements in the solder can lead the growth of the IMC.Apart from these influence,solder joints are also subjected to stress and strain caused by changes in thermodynamic conditions during service,which may result in the cracking failure of the IMC layer and the thermal fatigue failure of the solder joint.According to the result of finite element simulation and experimental,taking measures based on following two suggestions can improve the reliability of solder joint during soldring,isotherma aging,working or other thermodynamic conditions.The first suggestions is that choosing the right solder and substrate material controls the size,morphology,structure and evolution of the IMC to a certain extent.The other suggestions is take measures such as aging or heat dissipation to control the thermodynamic conditions of solder joints for reducing reliability problems caused by stress and strain. |
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